A flexible pressure sensor is for example described in document DE-A-42 37 072. This deals with a pressure detector to be integrated into the passenger's seat, which comprises two insulating flexible substrates disposed a certain distance one above the other with the aid of a spacer. The spacer comprises e.g. a double-sided adhesive tape which is cut out in such a way as to at least partly surround the active zones of the detector. Inside the active zones, one of the support sheets is furnished with two mutually separated electrode structures made of conducting material, while the other support sheet is furnished with a pressure sensitive coating of semiconducting or resistive material. The semiconducting or resistive material exhibits microbumps on the surface, so that the surface resistance between the layer and a conductor decreases when the layer is pressed down onto the conductor.
Another execution of a flexible seat sensor is described in document WO-A-00/26626. This deals with a sensor comprising a flexible support made of insulating material, at least two electrode structures disposed on said insulating substrate a certain distance from one another, and a layer of semiconducting material disposed above said electrode structures. Said layer of semiconducting material exhibits an internal resistance that varies with deformation of said layer and is disposed in intimate contact with the electrode structures.
In order to detect the presence of a passenger on a seat, the flexible sensor is integrated into the automobile seat, for example on the seating surface of the seat between the foam and the cover of the seat. The measured pressure and/or the distribution of the pressure over the seat make it possible to decide whether the seat is occupied and/or the stature of the seated person.
It is clear that detection devices of this kind need a defined reaction of the seat part situated under the flexible sensor in order to operate properly. This reaction of the seat ought to be constant over the lifetime of the vehicle and under any atmospheric conditions. However, the foam of the seat, for example a polyurethane foam, exhibits a hardness which is greatly dependent on the ambient temperature. Moreover, the stability of the foam will decrease with the age of the vehicle. It follows that the signal detected by such a detection device for one and the same occupancy of the seat will vary considerably in the course of time.